1. Technical Field
The present invention is related to a method and apparatus for rapid granulation of particles, and to the resulting product. More particularly, the present invention is related to the formation of granules or particles of a larger size or mass of a desired range from a feed stock of smaller particles.
2. State of the Art
There are numerous techniques available for granulation of particles. A general review of current methods of granulation can be found in Chapter 7 of the "Handbook of Powder Science & Technology", edited by Fayed et al. and published in 1984 by Van Nostrand Reinhold Co., New York.
Agglomeration or granulation of smaller or finer particles into relatively larger or bigger sized masses is desirable for several reasons. For instance, when microencapsulation or coating of fine particles is necessary to control the release or interaction of these particles into or with the surrounding medium or environment, it may be advantageous to have these fine particles compacted or clustered into larger sized masses so as to reduce the total interacting surface area. In other situations, the material is available only in smaller particle size than could be tolerated or used in a particular application or could be readily handled for further processing, e.g., filling capsules with a pharmaceutically active material in difficult to handle fine powdered form as originally synthesized. Other reasons for granulation may be related to esthetics, rheology or safety, e.g., preventing dust explosions or inhalation of toxic or allergenic materials, ease of conveying, prevention of caking, increasing or decreasing bulk density, facilitating removal of solids from liquids or gases, separation of one kind of solid from another by size differential after processing, etc.
The methods or apparatus heretofore employed for granulation or agglomeration and clustering of fine particles such as powdered materials suffer from certain limitations or disadvantages. Some devices, for instance, tablet presses have a lower size limit (about 1/8 inch) and a limit on the production rate (several thousand pieces per minute). Roll pressing and extrusion requires expensive precision equipment subject to wear and tear. Equipment of this nature may also have a product size limitation similar to that of tablet presses. Agitation methods include pan mixers, paddle mixers, inclined discs, rotating drums and the like. These methods as used by the prior art often suffer from low production rates and have high space requirements. On the other hand, those processes which employ spray drying tend to be expensive due to high energy cost and are typically used to form granules below 100 microns in diameter. Prilling is generally limited to materials stable in molten form and to particles above 1 mm in diameter. Fluid bed granulation has high space requirements and the energy costs are also high.
Of special concern is the ability to granulate particles of materials which are easily decomposed or degraded, or which become sticky for a period of time after being melted and cooled. Many materials have both a decomposition temperature and a melting point. Particularly difficult to deal with are materials which decompose or are otherwise deleteriously affected by temperatures close to, or at, the melting point. In general, however, it has been found that such decomposition results not only from reaching a particular temperature, but also from being maintained at that temperature for a particular length of time which may vary greatly from one material to another. Many prior art granulation techniques fail to recognize this critical concept, resulting in the inability to granulate certain materials with conventionally available techniques because the materials are maintained at the undesirable temperature for an excessive period of time. Frequently, for example, feed materials will be melted in their entirety and then fed as a liquid to a rotating disk or the like for centrifugal dispersion as in spray cooling. The resultant product from such techniques may be totally unacceptable in that it is "sticky" or otherwise difficult to handle for further processing, or important characteristics of the starting material may be adversely affected by such treatment.
Thus, a need for an improved, efficiently simple and cost effective granulation process and apparatus, and particularly one that is sufficiently fast to be capable of handling especially thermally sensitive materials, is quite apparent. This need is at least to an extent, met by a process and apparatus as disclosed in the parent application which generally improves upon the prior art techniques described above.
Thus, in accordance with the invention disclosed in the parent application, a feed material is provided which contains a material to be granulated in fine particulate form, which material is capable of being partially or completely melted for a short period of time without deleterious effect, or a particulate material to be granulated admixed with a particulate, meltable binder. The feed material is deposited onto central portions of the surface of a spreader, at least portions of which are maintained at a temperature which is at or above the melting point of the meltable component in the feed material. The spreader has peripheral portions which are spaced from the central portions in normal operation and also has inclined side portions between the central and peripheral portions thereof. In practical terms, the spreader may be likened to a concave or truncated inverted cone rotating element such as a dish or bowl. In operation of the process, the feed material rapidly spreads substantially radially outwardly on the spreader by centrifugal force produced by its rotation and with at least a portion of the feed material having a velocity component in the direction of the surface of the spreader to maintain this portion of the feed material in contact with the heated surface of the spreader as it moves radially outwardly so that it is melted substantially solely by contact with the heated surface to form a layer of liquid from the melted portion of the feed material on the surface of the spreader. The rate of feed of the material and the energy input to the surface of the spreader is adjusted along with the rotational speed of the spreader so that there is sufficient time for at least partial melting of the meltable component of the feed material but insufficient time to deleteriously affect the material being granulated. The material being granulated, including the liquid component, is discharged from the peripheral portions of the spreader into an atmosphere cooler than the melting point of the meltable component so that granules are solidified in the cooler atmosphere into granules which are larger than the particle size of the feed material.